A rigid, hinged-lid cigarette packet comprises a cup-shaped bottom shell or container and a top lid joined to each other by a hinge. When the lid is in a closed position closing the cup-shaped bottom container, the packet is parallelepiped-shaped and defined laterally by two parallel, opposite (respectively front and rear) major lateral walls, and by two parallel, opposite minor lateral walls; and, between the major lateral walls and the minor lateral walls, are defined four longitudinal edges, which may be square, bevelled, or rounded (as described, for example, in Patent EP-A-0205766).
A rigid, hinged-lid packet of the type described above is normally produced from a flat, substantially rectangular cardboard blank having a number of preformed longitudinal and transverse fold lines, along which the blank is folded to form the packet.
In some cases, the longitudinal edges of the finished rigid packets fall short of the desired shape, and the lateral walls (particularly the major lateral walls) are not perfectly flat, on account of the tendency of the folded blank to spring back to its original flat shape. Such defects are particularly evident when the longitudinal edges are other than square, i.e. are rounded or bevelled. To reduce springback of the blank, it has therefore been proposed to equip packing machines with flexing units, which perform a pre-folding operation to flex the blanks along the fold lines.
One example of a flexing unit is given in Patent EP-B1-0391118, which describes a packing machine comprising a linear conveyor for feeding the blanks to a folding wheel; and fixed flexing guides located on opposite sides of the linear conveyor to fold portions of, and so flex, each blank. At the end of the fixed guides, each blank springs back to a substantially flat shape, and is then fed to the folding wheel. The flexing guides, however, are fairly bulky, can make maintenance of the conveyor fairly difficult, fail to provide for precise flexing, and, above all, subject the blanks to damage by friction.
Another example of a flexing unit is given in Patent US-A1-4708704, which relates to a packing machine in which a blank is fed to a flexing station located upstream from a packing line and having movable folding members for folding portions of the blank along respective longitudinal fold lines and against respective contoured spindles. The blank is then allowed to spring back to the flat shape before being fed to the packing line. Apart from being fairly complicated and bulky, the flexing station described above has the major drawback of requiring that the blank remain stationary in the same position for a given length of time, and as such involves major complications for use on a continuous packing machine.
By way of an alternative to flexing the blank, it has been proposed, as for example in Patent EP-B1-0205894, to shape the blank in a die negatively reproducing the desired shape of the finished packet. More specifically, the blank is fed into alignment with the die, and is made to adhere to the inner walls of the die by a contoured mating die. Tests show, however, that flexing the blanks provides for better quality packets than die-shaping.